1. Field of the Disclosure
The present invention relates to a liquid crystal display (LCD) device, more particularly, to a photo-sensing type touch panel embedded liquid crystal display device that is able to sense touching stably, and a method for driving the same.
2. Discussion of the Related Art
Recently, as information age really starts, display technology for presenting electric information signal visibly has been developed rapidly and various thinner and lighter flat display devices having excellent power consumption efficiency have been developed correspondingly to replace the cathode ray tube (CRT) devices.
Such a flat display devices may include a liquid crystal display (LCD), plasma display panel (PDP), field emission display (FED) and electro-luminescent display device (ELD) device and the like.
Above devices include a flat display panel capable of presenting images as an essential element and such a flat display panel has a configuration in that a pair of transparent insulation substrate having a luminescent or planarizing material provided there between.
The LCD device among the devices can control light permittivity by using an electric field to display images. For that, the LCD device includes a display panel having a liquid crystal, a backlight unit emitting a light to the display panel and a driving circuit for driving the backlight unit and a liquid crystal cell.
The display panel has plural unit pixel regions defined by plural gate lines and data lines that crosses. Here, a thin film transistor array substrate and a color filter array substrate face each other, a space is provided between the two substrates to form a uniform cell gap, and a liquid crystal filled in the cell gap may be provided in each of the pixel regions.
The thin film transistor array substrate includes gate/data lines, a thin film transistor formed at each cross portion between the gate and data lines as a switching device, a pixel electrode formed of liquid crystal pixel units to be connected to the thin film transistor, and an alignment layer coated on the above elements. The gate and data lines receive signals from the driving circuits via pad parts for them, respectively.
Here, the thin film transistor responds to a scan signal supplied to the gate line and the gate line supplies a pixel voltage signal supplied to the data line to the pixel electrode.
The color filter array substrate includes color filters formed by the liquid crystal cell unit, a black matrix for identifying the color filters and for reflecting an external light, a common electrode for supplying a reference voltage to the liquid crystal cells commonly, and an alignment layer coated on the above elements.
The thin film transistor substrate and the color filter array substrate fabricated independently are aligned and then they are attached to each other. After that, liquid crystal is injected and sealed in a gap thereof.
Here, the liquid crystal display device is a passive light emitting device that may control brightness of a screen by using a light emitted from the backlight unit arranged in a backside of the liquid crystal panel.
Techniques for attaching a touch screen panel to such a liquid crystal display device have been suggested recently. The touch screen panel is a user interface typically attached to a display device to sense a corresponding touch point by changing an electrical characteristic at the touch point touched by an opaque material such as a user's finger or a touch pen. When the user's finger or the touch pen touches a screen thereof, the touch screen panel attached liquid crystal display device can detect corresponding touch point information to present various applications based on detected information.
However, such a liquid crystal display device would cause several disadvantages, such as a cost increase because of the touch screen panel, a yield decrease because of additional process to attach the touch screen panel to the liquid crystal panel, and brightness deterioration/thickness increase of the liquid crystal panel.
Thus, recently a photo-sensor has been formed in the display panel to control the backlight unit based on brightness of external lights and the touch screen panel has been tried to be formed in the display panel in order not to be attached to an outside of the display panel, which would only lead to an increase in the volume of the display panel.
As follows, a conventional liquid crystal display (LCD) device will be described.
FIG. 1 is a sectional view schematically illustrating a LCD device including a conventional photo-type touch panel and FIG. 2 is a diagram illustrating a circuit of a conventional touch sensor.
As shown in FIG. 1, an LCD device having the conventional photo-sensing type touch panel embedded therein includes a liquid crystal panel 10 including a first substrate 1, a second substrate 2, a pixel TFT 3 and a sensor TFT 4 and a backlight unit 20 provided under the liquid crystal panel 10 to transmit a light to the liquid crystal panel 10.
Here, to perform the photo-sensing, a light is emitted through the liquid crystal panel from the backlight unit 20 and the emitted light is reflected toward the liquid crystal panel, so that when a user's finger or a predetermined material 30 touches the liquid crystal panel, such is sensed by the sensor TFT 4.
In reference to FIGS. 1 and 2, such a conventional photo-sensing type touch panel embedded LCD device converts a digital video data into an analog data voltage with respect to a Gamma-reference voltage, and it supplies the converted voltage to data lines DL and a scan pulse to gate lines GL simultaneously, such that a data voltage is charged in a liquid crystal cell C1c. For that, a gate electrode of a pixel transistor Tpixel is connected to GL, a source electrode is connected to DL, and a drain electrode of the pixel transistor Tpixel is connected to a pixel electrode of C1c and to an electrode of capacitor Cst1. A common voltage Vcom is supplied to a common electrode of the liquid crystal cell C1c. A storage capacitor Cst1 has the data voltage applied from DL charged therein when the pixel transistor Tpixel is turned on to maintain the voltage of the liquid crystal cell C1c. If the scan pulse is applied to the gate lines GL, the pixel transistor Tpixel is turned on and a channel is formed between the source electrode and the drain electrode to supply the voltage in DL to the pixel electrode of the liquid crystal cell C1c. Here, alignment of liquid crystal molecules of the liquid crystal cell C1c is changed by an electric field formed between the pixel electrode and the common electrode such that incident lights may be modified.
A touch sensor includes a sensing transistor Tsensor for generating a different light power according to the amount of lights incident from outside, a sensing capacitor Cst2 for storing electric charge of the electric currents i and a switching transistor Tsw for switching output of the electric charges stored in the sensing capacitor Cst2.
A bias voltage Vbias preset below a threshold voltage thereof is supplied to a gate electrode of the sensor transistor Tsensor. A gate electrode of the switching transistor Tsw is connected to a front gate line GLn−1 and a source electrode thereof is connected to the sensing capacitor Cst2 and a drain electrode thereof is connected to the readout line RO.
A driving voltage line having a driving voltage Vdrv applied there through is connected to a drain electrode of the sensor transistor Tsensor.
Such a touch sensor detects touch by using difference of photoelectric currents readout from the readout line RO in cases of a touch and a non-touch.
Operation of the touch sensor will be schematically described as follows.
The light reflected in case of a touch increases conductivity of the sensor transistor Tsensor enough to cause flow electric current toward the sensing capacitor Cst2 from the bias line Vbias, and a value beyond an initial voltage below the bias voltage Vbias is stored in the sensing capacitor Cst2 according to a touch condition.
Hence, if the switching transistor switch TFT connected to the readout line RO is turned on, the electric charge stored in the sensing capacitor Cst2 is output to the readout line RO to sense whether there is a touch and the sensing capacitor Cst2 is reset to a reference voltage value. After the reset, sensing for the touch may be performed and touch sensing is performed according to the identical process.
FIG. 3 is a diagram illustrating sensing degrees in comparison to a reference value according to the amount of external lights in cases there is a partial touch.
As shown in FIG. 3, as a point of a surface of the liquid crystal panel 10 is touched, a sensing degree of the touch point T is differentiated according to the light amount strength of external lights.
To sense touching, a predetermined reference value Ref is preset and change of photoelectric current strength of the backlight reflected light read at the touch point may be detected.
For example, if a point is touched in case of a strong external light, the light amount at the touch point is small in comparison to the reference value. As the difference between the light amounts at the touched point and the non-touched area is relatively large with respect to the reference value Ref, the touch detection may be performed smoothly based on the light amount difference.
In case of a middle external light, the light difference between the touched point and the other non-touched area is substantially not much larger and this light difference is not much larger with respect to the reference value ref. As a result, it is difficult to detect the touching because of the little difference.
In case of a weak external light, the light amount at the non-touched area is almost identical to the reference value Ref. In this case, the touched point has a relatively large value of the light amount with respect to the reference value. However, there is little difference of the light amount between the touched point and the non-touched area, and thus, it is difficult to detect the touching also.
However, the above conventional touch panel in-cell type LCD device may have following disadvantages.
First, according to the conventional touch panel embedded LCD device, the external light as well as the light reflected from the backlight may be recognized. As a result, the sensor may fail to recognize the input signal precisely if the illumination intensity of the reflected light is similar to that of the external light.
Furthermore, as the output signal according to a touch is differentiated according to external environment, algorithm manufacturing for touch point recognition may be complicated only to cause malfunction of the product. That is, if the external light is higher than the reflected light, the touch signal is output with a voltage lower than voltages of the other area. If the external light is lower than the reflected light, the touch signal is output with a higher voltage. As a result, it is difficult to recognize an actual touch signal.
Still further, if the sensor transistor is degraded, the touch sensor might output a non-touch sensing signal even when the user's finger or the like touches the liquid crystal cell formed thereon, and vice versa. The touch panel in-cell type LCD device may sense the touch point only with the relative difference of photoelectric current flowing via the sensor transistors. In this case, if the sensor transistor is degraded, it is almost impossible to recognize the actual touch precisely. For example, in case external environment has a strong illuminance, a shadow caused by the external light with the user's finger not touching the liquid crystal panel may be recognized as a touch.